The present invention relates to paper machines.
In particular, the present invention relates to structure for damping pressure and flow rate disturbances in pulp suspension flowing toward a headbox.
Thus, the structure of the present invention is intended to be mounted in the pulp pipe system which delivers the pulp suspension to the headbox of the paper machine.
As is well known, disturbances will unavoidably occur in the pulp suspension flowing in an approach pipe system of a paper machine. Thus the pulp stock flows through this approach pipe system toward a headbox such as a hydraulic headbox. With respect to such disturbances, the situation is ideal in the event that each longitudinal element at the lip slice of the headbox continuously discharges precisely the same quantity of suspension per unit of time at a constant velocity. In the event that the rate of flow is the same over the entire breadth of the slice, but varies with respect to time, then there will be a dry weight variation in the machine direction in the paper manufactured thereby.
On the other hand, if the pulp suspension flow is constant with respect to time but varies depending upon the particular location in the cross-machine direction, then a transverse dry weight variation will occur in the paper. This latter type of variation cannot be eliminated by way of the present invention nor by any other damping systems located in the approach pipe system of the stock supply. It is well known that the adjustment of the profile in the cross-machine direction, which is the problem in this particular case, is carried out by way of fine adjustment spindles at the lip slice of the headbox.
Briefly, the output disturbance signals with which the present invention is concerned are in the form of dynamic pressure variations at the lip flow aperture, while input disturbance signals are derived from a number of different sources such as variation in hydrostatic pressure in the pipe system, variation in the output pressure of the pump, variation in the pressure drop of the flowing suspension, pulse pressures due to vibrations transmitted to the pipe system through its supports, and pressure variations caused by turbulence vortices in the pipe system, particularly at the location of valves, pipe bends, etc. It has been found in practice that the different disturbance signals each have their own specific, frequently rather wide frequency spectrum. However, the disturbance signals from pumps, for example, have spectra characterized by distinctly absorbable peaks at the frequencies which are consistent with the speed of rotation of the respective pump and with its multiplesand subharmonics.
In general, paper machine headboxes may be divided into three main groups:
(a) headboxes provided with an air cushion forming a part of the headbox, or so-called air cushion headboxes, PA1 (b) hydraulic headboxes provided with an air cushion and mounted separately from the headbox itself, wherein air tanks are located either in the approach pipe system for the paper stock suspension in advance of the distribution header or subsequent to the distribution header, and PA1 (c) hydraulic headboxes which do not have any air cushions.
The air cushions are normally used in connection with headboxes in an attempt to equalize pressure variations occurring in the pulp suspension flow prior to the discharge aperture or lip slice of the headbox. These variations may originate in the pulp stock system preceding the headbox or in the headbox itself.
In an air cushion headbox according to type a) referred to above, there is usually an efficient damping of pressure variations with respect to time, inasmuch as the surface area of the pulp stock contacting the air cushion is relatively large while the height of the pulp stock, measured perpendicularly to its direction of flow, is relatively small. A further advantage of such headboxes resides in the fact that the air cushion usually extends up to the vicinity of the discharge slice, so that there is little opportunity for new pressure variations to be generated in the flow between the air cushion and the lip slice.
However, even though the above type of construction has the above favorable features, these air cushion headboxes have in recent times yielded, particularly in the most modern fast paper machines, to hydraulic or fully hydraulic headboxes of the types (b) and (c) referred to above. This development has occurred because the latter two types of headboxes are easier to utilize and situate in connection with the relatively new twinwire formers, and in addition such structures have lower manufacturing costs. The greater turbulence of the pulp stock jet discharging from the lip and its more favorable intensity distribution, as well as the better homogeneity of the stock resulting, are also factors which favor the use of these hydraulic headboxes.
As opposed to these advantages, however, hydraulic headboxes have presented certain difficulties as a result of the pressure variations referred to above. Thus in many instances it has been necessary to provide a headbox initially meant to operate as a fully hydraulic headbox with one or more separate air tanks which tend to be a substitute for the air cushion in an air cushion headbox. Various designs are known with respect to the situation of such separate air tanks. Thus in some designs these air tanks are connected to the pulp stock pipe system in advance of the headbox, while in other designs these air tanks are situated above the headbox itself and connected to the upper part of the headbox by suitable connecting tubes or by a connecting duct.
However, these latter designs have a serious drawback in that an air tank situated above the headbox necessitates a relatively great height for the free liquid level over the central axis of liquid flow, or the communicating tubes or duct from the headbox to the air tank must be dimensioned in such a way that they are relatively narrow as compared with the main flow duct. In either case the damping capability is impaired, as contrasted with the pressure variation damping capacity of a normal air cushion headbox.